Magazining dwell pressures during injection moulding - using hydraulic circuit, to shorten cycles by concurrent plasticisation for the next shot

摘要

An ancillary hydraulic system for a screw injection moulding machine to maintain the packing or dwell pressure on the melt entering the mould, and allowing the screw to retract and plasticise more material concurrently with the dwell stage of injection. A conventional reciprocating a non-return valve. Downstream from the valve is a small side chamber contg. a plunger. The plunger is moved by a hydraulic ram on a variable pressure circuit interconnected with that for the ram for the main screw. The relative ram forces can be adjusted so that during mould filling with full pressure on the screw, (a) the plunger can be advanced sufficiently to completely purge the side passage but (b) at the end of the initial shot can be retracted to allow the screw to fill the side passage. The ancillary circuit can then be isolated to maintain a suitable mould packing dwell pressure via the ancillary plunger while the non-return valve remains closed to allow the screw to retract etc. The plunger cam pressure can be relieved to allow the tool to be opened and the moulding ejected without any pressure on the melt in the runners and mould gates. Esp. suitable for moulding crystalline or semicrystalline materials, viz certain polyolefins, to compensate for contraction within the material in the mould cavity. The packing pressure is readily set to optimum levels. Overall mould cycles are reduced significantly by performing plasticisation for one cycle concurrently with the dwell stage of the preceding cycle. There are no blind recesses in the flow paths in which moulding material could hang up and degrade.